1. Field of the Invention
The present invention relates generally to the field of agriculture and plant genetics. More particularly, it concerns genetically modified plants comprising reduced lignin content.
2. Description of Related Art
Ethanol is increasingly being considered as a renewable, cleaner alternative to petroleum based fuels. Ethanol may also be produced from sugars and starches as well as from lignocellulosic based biomass which constitute the most abundant biomass on earth. While energy conversion from lignocellulosic biomass is less efficient, it does not require crops used for human food production (e.g., corn) to be used as feed stock.
The principal source of fermentable sugar in lignocellulosic biomass is cellulose. In typical lignocellulosic biomass used for ethanol production, cellulose accounts for between 35 to 50% of the mass. Cellulose is a long chain polysaccharide carbohydrate, composed of repeating cellobiose (β-1,4 glucose disaccharide) units. Hemicellulose also contributes to the fermentable sugar content of lignocellulosic biomass. It comprises about 20 to 35% of lignocellulosic biomass mass, and is a mixture of a variety of sugars including arabinose, galactose, glucose, mannose, and xylose, and derivatives of such sugars.
The third major component of lignocellulosic biomass, lignin, is not a sugar based fermentable polymer. Lignin is a complex polymer of hydroxylated and methoxylated phenylpropane units, linked via oxidative coupling and comprises about 12 to 20% of lignocellulosic biomass. For ethanol production from lignocellulosic biomass, the cellulose and hemicellulose components are processed to produce their constituent sugars, and these sugars are then used to make ethanol via fermentation. However, lignin does not contribute fermentable sugar to lignocellulosic biomass, and its presence reduces the efficiency of enzymatic hydrolysis of cellulose, apparently by physically shielding the cellulose molecules from the hydrolytic enzymes. Consequently, chemical loosening of lignin from the lignocellulosic biomass is often one of the first steps in the ethanol production processes. This process consumes energy, and utilizes chemical treatments (e.g., hot acid) that require clean-up (e.g., neutralization and disposal of waste).
Genetic modifications necessary to achieve reduced lignin content and improved cellulose and/or hemicellulose availability for saccharification were not clear based on currently available information. Development of plants with modified cell wall composition would have a significant benefit for the production of ethanol from plants and could potentially have a broad range of other beneficial applications.